This invention relates to a compact disc pickup drive mechanism and, more particularly, to a direct drive mechanism having a mounting system which helps to avoid problems due to misalignment of the motor and driving mechanism.
A compact disc player receives audio information in the form of digital data stored in a spiral fashion on an optical disc. A laser pickup is used to "read" the information on the disc and transmit it to circuitry which translates this data into an audio signal. The laser pickup travels parallel to the disc along a radial path. The position of the pickup along the face of the disc dictates what information is being "read." The speed and location of the pickup should be precise to insure that the appropriate information is being read at the proper time.
Two different compact disc pickup linear drive systems are commonly used. The first is a rack and pinion drive, wherein a rack affixed to a laser pickup is moved by a motor driven pinion. A rack and pinion drive system responds to the rotation of a series of gears, each of which introduces an error or delay.
The second type of linear drive mechanism is a screw and nut system. Here, the laser pickup is attached to a nut which is driven by a screw turned by a motor. The screw and nut drive is preferable because the number of parts between the motor and the pickup is reduced to one, the screw. Also, the screw and nut system is more accurate because error introduced by each gear is minimized. The absence of gears improves the reliability of tracking because the pickup moves in direct response to the motor without the introduction of positioning error (backlash) from a gear drive system. The screw and nut drive system provides a smoother and more consistent drive load.
Backlash is an inherent problem with any gear drive system. Backlash is caused by the clearance provided between the engaged teeth of two gears. The clearance provides smooth engagement of the gears, but also introduces a positioning error of one gear with respect to the other. The backlash between two gears creates a delayed rotational displacement of one in response to a driven rotational displacement of the other. One disadvantage of having the problem of backlash in a drive gear system for the pick-up carriage of a compact disc player is that the pick-up may continue to move a small (yet appreciable) amount after the drive motor stops thereby misplacing the pickup and affecting the operation of the cd unit. The positioning error may be detected by the microprocessor of the unit which would cause the motor to reverse the pick-up carriage and correct for the error. The correct position would eventually be reached but only after reversing the motor at least once. If, because of the backlash of the gear drives, it becomes necessary to correct the position of the pick-up for every position request, it is likely that the useful life of the motor will be reduced considerably. Also, such repeated reversals of the motor may cause the microprocessor to shut down the entire unit, reducing its apparent quality and operative performance.
Systems where the drive motor is directly connected to the screw are disclosed in U.S. Pat. No. 4,825,432 to Takahashi and U.S. Pat. No. 4,003,059 to Sugiura et al. Placing a motor in a direct drive configuration with the drive screw improves response time between the signal reception by the drive motor and the motion of the laser pickup. This is because the pickup responds to the rotation of the screw, which is directly coupled with the motor.
Although the screw and nut drive system is preferable to the rack and pinion drive, its practical use has been limited. In order for a direct drive system to work reliably, the pickup must start and stop at precise predetermined locations and the screw must turn at a uniform speed. The first requirement is met if the motor reacts instantaneously to the application or removal of power; the second requirement is met if the power required to turn the screw is uniform at all positions of the screw. The uniform power requirement necessitates precise alignment. Otherwise, misalignment will cause an uneven load on the motor, causing the screw to slow down or speed up at various points in its rotation. However, it is not economically feasible to adjust the alignment of each individual product when produced in huge quantities. Accordingly, the prior art would commonly align the shafts with a flexible coupling. The coupling is flexible to accommodate misalignment between the affixed motor and the affixed drive screw. However, the flexible coupling is unsuitable for a laser pickup direct drive system because it adds a changing, flexing load when the shaft is a misaligned. This will cause the motor to experience changing load patterns as the shaft rotates which, in turn, reduces the drive speed at some positions and increases it at others.